Control Strategies for Linear Synchronous Motors

Linear motors present both advantages and challenges when utilized to control vehicles on a guideway. Linear motors are superior to other means of propulsion in that force is applied directly to the vehicle, does not depend upon friction, is tightly controllable, and the linear motors themselves have no wearing parts. Use of linear motors does, however, present some unique challenges. The back EMF can vary with position as well as speed, the overall thrust constant can vary if there are gaps between the stators, and stators must be synchronized as a vehicle transitions from one to the other. Strategies for successfully managing these characteristics will be discussed. One of the key linear motor design parameters is the length of the stator that is energized, referred to here as block length. There are a wide range of strategies for sizing block lengths, ranging from individually driving each pole to driving kilometer long blocks. Tradeoffs for block length in terms of cost, efficiency, throughput, and reliability will be discussed along with specific strategies for one application. Methods for operating multiple vehicles safely under the constraints of linear motor block-based control will also be discussed.